If a Hopper pipe is routed over a Chest where items drop into it, and a Hopper pipe is routed under the Chest to catch the items passing through the Chest, sometimes the items pass through the chest without triggering a Redstone Comparator that is attached to the Chest.

How to reproduce (see screenshot for clarification):
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Place a Chest and put a Hopper over it. Put a Redstone Comparator against the Chest so it turns ON when an item is placed in the Chest. Put a Hopper under the Chest, and the item will drop into it and turn OFF the Redstone Comparator. Add several Hoppers against the bottom Hopper to create a pipe so the item ends up in the last Hopper.

If you place an item in the upper Hopper, sometimes the Redstone Comparator will flash indicating something passed through the Chest before the item drops into the Hopper below it, and sometimes it won't. The item will always pass through to the last Hopper.

If the Redstone Comparator does NOT flash when an item is placed in the upper Hopper, try putting an item directly into the Chest and the Redstone Comparator will most likely flash. After that, the Redstone Comparator will most likely flash when an item is put into the upper Hopper. But if you despawn the chunk where this structure is located (go far away and come back), the Redstone Comparator will not flash until the Chest is accessed directly again.

Gotchas:
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• Sometimes it works as built and sometimes it does not, apparently location has something to do with whether it works or not. You will have to build several structures until you reproduce the bug.

• Sometimes when it doesn't work, directly accessing the chest still won't make the Redstone Comparator work.

• If you build it and it doesn't work, and then it works after directly accessing the chest, teleport far away to despawn the chunk and teleport back, and it won't work again until the chest is accessed again.

• I've been able to reliably reproduce the bug by building the structure where the Redstone Comparator is placed 'west' of the Chest.

• I've tried replacing Chests with Dispensers, and the structure operates with the same bugs; if it worked with the Chest, it will work with the Dispenser. If it didn't work with the Chest, then it won't with the Dispenser. The only difference is that it will NOT work if you first access the Dispenser and drop an item directly into it.

• I've been able to reliably reproduce three different scenarios, and it all depends solely on where the structure is built: 1) It works as expected. 2) It doesn't work until the chest is accessed first, then it works fine until the chunk is despawned. 3) It doesn't work at all, nothing makes the Redstone Comparator work.